states this article at the WSJ. While I have some reservations as to the criteria (Hong Kong, the world’s freest economy?), the article points out this:

Most alarming is the U.S.’s fiscal burden, which imposes high marginal tax rates for individuals and very high marginal corporate tax rates. In terms of corporate taxation as an element of economic freedom, the U.S. ranks a lowly 112th out of the 155 countries scored, and its top individual tax rate ranks only slightly better at 82nd. U.S. government expenditures as a share of GDP increased less in 2003 than in 2002, but the rise since 2001 is what explains the U.S.’s decline in score over the period.